牛客 - 练习赛67F 牛牛的苹果树 （区间直径）

传送门：牛客 - 练习赛67F

思路分析

两个区间内的直径有传递关系，可以通过两组端点两两组合得到

AC代码

#include <bits/stdc++.h>
#define fi first
#define se second
#define ll long long
#define pb push_back
#define mp make_pair
#define fun function
#define sz(x) (x).size()
#define lowbit(x) (x)&(-x)
#define all(x) (x).begin(),(x).end()
#define mem(a,b) memset(a,b,sizeof(a))

namespace FastIO {
#define BUF_SIZE 100000
#define OUT_SIZE 100000
    bool IOerror=0;
    inline char nc() {
        static char buf[BUF_SIZE],*p1=buf+BUF_SIZE,*pend=buf+BUF_SIZE;
        if(p1==pend) {
            p1=buf;
            pend=buf+fread(buf,1,BUF_SIZE,stdin);
            if(pend==p1) {
                IOerror=1;
                return -1;
            }
        }
        return *p1++;
    }
    inline bool blank(char ch) {
        return ch==' '||ch=='\n'||ch=='\r'||ch=='\t';
    }
    template<class T> inline bool read(T &x) {
        bool sign=0;
        char ch=nc();
        x=0;
        for(; blank(ch); ch=nc());
        if(IOerror)return false;
        if(ch=='-')sign=1,ch=nc();
        for(; ch>='0'&&ch<='9'; ch=nc())x=x*10+ch-'0';
        if(sign)x=-x;
        return true;
    }
    template<class T,class... U>bool read(T& h,U&... t) {
        return read(h)&&read(t...);
    }
#undef OUT_SIZE
#undef BUF_SIZE
};
using namespace std;
using namespace FastIO;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

const int INF = 0x3f3f3f3f;
const int N = 1e6+10;
#define int long long
vector<pair<int,int>>e[N];
int fa[N],siz[N],d[N],son[N],top[N],dep[N];

void dfs1(int u,int f) {
    siz[u]=1;
    fa[u]=f;
    dep[u]=dep[f]+1;
    for(auto x:e[u]) {
        int v=x.fi;
        int w=x.se;
        if(v==f) continue;
        d[v]=d[u]+w;
        dfs1(v,u);
        siz[u]+=siz[v];
        if(siz[v]>siz[son[u]]) son[u] = v;
    }
}

void dfs2(int u,int t) {
    top[u]=t;
    if(!son[u]) return ;
    dfs2(son[u],t);
    for(auto x:e[u]) {
        int v=x.fi;
        int w=x.se;
        if(v==fa[u] || v==son[u]) continue;
        dfs2(v,v);
    }
}

int lca(int x,int y) {
    while(top[x]!=top[y]) {
        if(dep[top[x]]<dep[top[y]]) swap(x,y);
        x=fa[top[x]];
    }
    return dep[x]>dep[y]?y:x;
}

int dis(int x,int y) {
    return d[x]+d[y]-2*d[lca(x,y)];
}

struct node {
    int l,r,x,y,w;
} tree[N<<2];

#define ls x<<1
#define rs x<<1|1
#define mid (tree[x].l+tree[x].r>>1)
node merge(node a,node b) {
    node ans = (a.w>b.w?a:b);
    int tmp = dis(a.x,b.x);
    if(tmp>ans.w) {
        ans.w=tmp;
        ans.x=a.x;
        ans.y=b.x;
    }
    tmp = dis(a.x,b.y);
    if(tmp>ans.w) {
        ans.w=tmp;
        ans.x=a.x;
        ans.y=b.y;
    }
    tmp = dis(a.y,b.x);
    if(tmp>ans.w) {
        ans.w=tmp;
        ans.x=a.y;
        ans.y=b.x;
    }
    tmp = dis(a.y,b.y);
    if(tmp>ans.w) {
        ans.w=tmp;
        ans.x=a.y;
        ans.y=b.y;
    }
    return ans;
}

void build(int x,int l,int r) {
    tree[x]= {l,r,0,0,0};
    if(l==r) {
        tree[x].x=tree[x].y=l;
        return ;
    }
    build(ls,l,mid);
    build(rs,mid+1,r);
    tree[x] = merge(tree[ls],tree[rs]);
    tree[x].l=l;
    tree[x].r=r;
}

node query(int x,int l,int r) {
    if(l<=tree[x].l && tree[x].r<=r) {
        return tree[x];
    }
    if(r<=mid) return query(ls,l,r);
    else if(l>mid) return query(rs,l,r);
    else return merge(query(ls,l,r),query(rs,l,r));
}

signed main() {

#ifdef xiaofan
    freopen("1.in","r",stdin);
    freopen("1.out","w",stdout);
#endif

    int n,q;
    read(n,q);
    for(int i=1;i<n;i++){
        int u,v,w;
        read(u,v,w);
        e[u].pb(mp(v,w));
        e[v].pb(mp(u,w));
    }
    dfs1(1,0);
    dfs2(1,1);
    build(1,1,n);
    while(q--){
        int l,r;
        read(l,r);
        printf("%lld\n",query(1,l,r).w);
    }




    return 0;
}